Power saving in communication terminals

ABSTRACT

A communications terminal using interleaving may be awakened at certain times. The paging message determines whether the terminal will remain awake or it can return to sleep. The amount of time that the communications terminal is awakened may be reduced by determining whether it is necessary to demodulate all of the available time slots. In many cases, it may be possible, based on the environment in which terminal finds itself and the quality of the signals it receives, to obtain the necessary information without demodulating symbols from all the time slots in which symbols of the message are transmitted. As a result the period to receive the paging message may be reduced and the power consumption may likewise be reduced.

INCORPORATION BY REFERENCE

This application is a Continuation of application Ser. No. 09/962,424,“Power Saving in Communication Terminals” filed on Sep. 25, 2001. Thedisclosure of the prior application is incorporated herein by referencein its entirety.

BACKGROUND

The present invention relates to communication terminals such ascellular phones and pagers and in particular to methods for receivingand decoding messages by communication terminals.

Messages transmitted to cellular phones are encoded in order to reducethe chance that noise will corrupt the message. The messages comprise aplurality of bits, which are either ‘0’ or ‘1’. The encoding of themessage usually includes adding redundancy (i.e., making the encodedmessage longer than the original message), introducing symbol dependency(i.e., making the value of each symbol in the encoded message a functionof a plurality of bits in the original message) and interleaving themessage (i.e., mixing the order of the symbols in the encoded message).

Normally, each symbol in the received message is represented in thereceiver, before decoding, by a word, which represents the chance thatthe symbol is a logical ‘1’. A high positive value of the word meansthat the symbol is a logical ‘1’ with high confidence, while a negativevalue with a high absolute value means the symbol is a logical ‘0’ withhigh confidence. A word with a zero value means that the symbol is a ‘0’or ‘1’ with equal probability. This representation is referred to hereinas soft data.

The decoding of the message is usually performed by finding an originalmessage, which would result in the received encoded message with thehighest probability. Due to the interleaving and the interdependence ofthe symbols of the encoded message, the decoding process does notusually begin before the entire message, or an entire frame of apredetermined length, is received. If the message were not interleaved,some decoding schemes would allow the decoding to begin before theentire message was received and end when the entire message is received.However, such schemes usually have a reduced performance, i.e., a higherrate of failure in correctly decoding the message, in particular infading channels as often encountered in practice.

The messages usually include an error detection code, which is used todetermine whether the message was properly decoded. One such code is thecyclic redundancy code (CRC).

Most cellular phones operate on rechargeable batteries. Some cellularsystems reduce the rate at which the cellular phones consume batterypower by having the phones operate in an idle mode when they are openfor receiving calls but are not transmitting or receiving data.Generally, in the idle mode, the cellular phones deactivate most oftheir components to reduce their current consumption. Periodically, forexample, once every two seconds, the cellular phones activate all theircomponents, for a short wake up period during the idle mode. A basestation of the system sends a broadcast or paging message to thecellular phones on a paging channel, notifying them to remain in theidle mode or to change to a reception mode, for example in order toreceive an incoming call.

The wake up period of a cellular phone comprises a warm-up period, areception period, a decoding period and a shut down period. In the wakeup period the cellular phone activates all of its components in order toreceive the message. If the message tells the cellular phone to remainin the idle mode, the cellular phone shuts down most of its componentsduring the shut down period. Any reduction in the length of the wake upperiod results in an increase in the time a cellular phone may be usedwithout recharging or replacing its battery.

Thus, there is a need for better ways to limit the amount of powerconsumption during the receipt of an interleaved paging message.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustration of a receiver, inaccordance with one embodiment of the present invention;

FIG. 2 is a flow chart in accordance with one embodiment of the presentinvention;

FIG. 3 is a flow chart in accordance with one embodiment of the presentinvention;

FIG. 4 is a graph of simulation results showing the average number ofreceived slots versus the signal to noise ratio (SNR) for a staticchannel; and

FIG. 5 is a graph of simulation results showing the average number ofreceived slots versus the signal to noise ratio for a plurality offading channels.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1, a receiver 20 comprises a de-modulator 22 thatreceives frames of transmitted messages via an antenna 24, and ade-interleaver 26 that stores an incoming frame. A decoder 28 decodesthe incoming frames from the de-interleaver 26. After accumulating anentire message, which may include a plurality of frames, a single frameor less than a single frame, a CRC checker 30 checks the decoded messagefrom the decoder 28 to determine whether the decoding was successful.

The receiver 20 also includes control circuitry 44, implemented forexample by a digital signal processor (DSP), with appropriate software,that instructs the decoder 28 when to begin decoding a received frame.In order to save time, at least some of the frames received by receiver20 may be decoded before the de-modulator 22 receives the entire frame,and the decoding may be performed without received values for at leastsome of the symbols of the frame. Further, the decoding may finishbefore the entire frame is received in one embodiment.

The control circuitry 44 may indicate which data to pass to the decoder28 from the de-interleaver 26 for decoding. The control circuitry 44 maydetermine when to perform decoding based on an input from a clock 32.

The components of receiver 20 shown in FIG. 1 may be implemented by aplurality of sub-components or may be part of a single physicalcomponent, which includes other components. For example, in someembodiments of the invention, the filler unit 27 is an integral part ofdecoder 28 and/or of de-interleaver 26.

In the idle mode, the receiver 20 wakes up periodically, atpredetermined times, in order to receive a control message from a basestation. The control message informs the receiver 20 whether thereceiver 20 is to receive an incoming message or return to sleep. Thecontrol message is typically shorter than the length of a single frameand is included within a single frame. The handling of the pagingmessages is responsible for most of the power consumption of receiver 20during the idle mode. Thus, it may be advantageous in some embodimentsto reduce the average length of the period when the receiver 20experiences increased idle mode power consumption.

When a message from the base station to receiver 20 is due, clock 32, ora different clock controlling the receiver 20, wakes up the receiver 20.The de-modulator 22 begins to receive a frame and store it in thede-interleaver 26.

In one embodiment of the present invention, the decoder 28 decodesconvolutional codes with a Viterbi style decoder, as described, forexample, in chapter 6.2 of “Error-Correction Coding for DigitalCommunications,” by G. C. Clark and J. Bibb Cain, Plenum Press, March1988 and in Forney, G. D., Jr. (1972), “Maximum-Likelihood SequenceEstimation of Digital Sequences in the Presence of IntersymbolInterference,” IEEE Trans. Inform. Theory, Vol. IT-18, pp. 363-378.

On the transmission side from the base station, for example, a block maybe transmitted over four time slots. In one embodiment of the presentinvention, a Global System for Mobile Communications (GSM) protocol maybe utilized. See GSM Specification TS 101 220 (2000-05-26) Version 8.00“Digital Cellular Telecommunications System (Phase 2+)” available fromthe European Telecommunications Standards Institute, 06921 SophiaAntipolis, France. In such an embodiment, the paging information maycomprise 184 bits, a check sum may comprise 40 bits, and four zeros maycomprise 4 bits, making a message of 228 bits used for convolutionalcoding. In such an embodiment, at the transmitter, the 228 bits arepassed through a one-half rate convolutional code so that for each bitof information, two symbols come out of the encoder. Thus, 456 symbolsare interleaved and transmitted over four time slots in one embodiment.Each time slot includes 114 symbol positions in that embodiment.

The de-modulator 22 extracts the transmitted symbols. The de-interleaver26 receives 456 symbols in one embodiment. In interleaving, symbols maybe placed in a plurality of different time slots, such as four timeslots. As one example, the first of 456 symbols may be placed in a firsttime slot, the second symbol in the second time slot, the third symbolin the third time slot, the fourth symbol in the fourth time slot, thefifth symbol in the first time slot, the sixth symbol in the second timeslot, the seventh symbol in the third time slot and the eighth symbol inthe fourth time slot, and so on. Thus, successive sets of four symbolsare placed successively in the four time slots.

Adjacent symbols are spread across different time slots. By spreadingthe symbols across the time slots, it is more likely that those symbolsmay be recovered. The de-interleaver 26 may recover the symbols from thedifferent time slots in the correct order. The decoder 28 thencalculates the check sum and the checker 30 checks the check sumcalculated by the decoder 28 to determine whether or not it is correct.

The filler unit 27 may not be used in some embodiments. Whenever symbolsare missing because they could not be demodulated, a zero may beinserted by the filler unit 27 where the symbol would otherwise havebeen.

Referring to FIG. 2, at predetermined intervals, the receiver 20 wakesitself up. In some embodiments, the receiver 20 knows when it is goingto receive a paging symbol and automatically wakes itself up insufficient time to receive the message. For example, the network maytell the receiver 20 when the paging messages will be transmitted. Thereceiver 20 demodulates the symbols in the first two time slots asindicated in block 10. If the symbols are decoded and are correct, asdetermined in diamond 12, then the receiver 20 may go back to sleep asindicated in block 14 assuming there are no other incoming messages. Ifthe data is not good, then the receiver may continue to demodulate thenext ensuing time slot as indicated in block 16. As a result, if themessage is very clear, it may not be necessary to demodulate all of thesymbols in all of the time slots, reducing the amount of time involvedin receiving the paging message. As a result, the amount of powerconsumed in the idle state may be reduced.

Turning next to FIG. 3, the power saving software 46 may be stored, forexample, in association with the control circuitry 44 in one embodiment.The software 46 may begin by determining whether the paging message timehas arrived as indicated in diamond 48. If so, the receiver 20 may beawakened as indicated in block 50 to receive the message. The receiver20 may receive and demodulate the symbols of the first and second timeslots as indicated in blocks 52 and 54.

In block 56, slot quality data may be obtained. In one embodiment of thepresent invention, the noise data may be carrier to interference ratio(CIR) data. If the signal is sufficiently low in noise, the first twotime slots may be decoded as indicated in block 60. Next, an error codemay be checked as indicated in block 62. If the received data isacceptable, as determined in diamond 64, a check at diamond 66determines whether there is another incoming message such as thetelephone call. If so, that call is handled as indicated in block 68. Ifnot, the receiver 20 can return to the sleep mode as indicated in block70 wherein power is conserved.

If at diamond 58 it is determined that the signal is too noisy, theoperations of block 60 and 62 may be skipped and the flow may continueto demodulate the next time slot as indicated in block 72. In such case,it has been determined that the signal is too noisy and therefore itwould be a waste of power to proceed with decoding when the odds ofsuccess are so low. As a result, power consumption may be preserved byavoiding unnecessary mathematical decoding operations.

Similarly, if, as a result of the decoding, it is determined that thedata from the first and second time slots is not sufficient, the flowproceeds to demodulate an ensuing time slot. In such case, the decodingof an ensuing time slot increases the power consumption, but in suchcase, it is necessary to proceed in this fashion.

For example, as indicated in FIGS. 4 and 5, based on simulations, itappears that, in general, power can be conserved in both static channel(FIG. 4) and fading channel (FIG. 5) environments. For example, FIG. 4shows a static channel configured according to the GSM specification.With reasonable signal to noise ratios, it is often possible to receiveand demodulate less than all four available time slots. This necessarilycorresponds to significantly less power consumption. Likewise in FIG. 5,with a fading channel, the average number of time slots is stillsignificantly lower than four, indicating that at a range of signal tonoise ratios, the need to demodulate all four time slots may be avoided,resulting in power savings.

In FIG. 5, a typical urban signal where the mobile is traveling at 50miles per hour in a non-frequency hopping mode (TU50NH), a rural areamobile traveling at 250 kilometers per hour in a non-frequency hoppingmode (RA250NE) and a hilly terrain mobile traveling at 100 kilometersper hour in a non-frequency hopping mode (HT100NH) are all illustrated.Thus, FIG. 5 indicates that in a variety of fading channel environments,power may be saved by using techniques in accordance with embodiments ofthe present invention.

While an embodiment has been described in connection with a GSM cellulartelephone, those skilled in the art will appreciate that embodiments ofthe present invention may be applicable in a variety of communicationsnetworks including wireless and wired communication networks that useinterleaving. For example, the present invention may be used withpagers, optical networks, and wireless networks including radiofrequency and infrared networks, to mention a few examples.

In some embodiments of the present invention, determining whether themessage was successfully decoded is performed using methods other thanCRC in addition to, or instead of, using the CRC check. Such methodsinclude, for example, other error detection codes such as parity checksand block code checks, and a comparison of the received code to a closedgroup of possible messages.

It is noted that in some embodiments of the present invention, the CRCis used to correct errors in addition to detecting errors. In theseembodiments a decoding is treated as having failed only if the CRC wasnot able to correct the error. That is, if the CRC check failed but theerror was corrected, the decoding is considered successful.

Although in the above description an example was giver in which thedecoder 28 operates on Viterbi style codes, the decoder 28 may operatein accordance with substantially any other code, such as turbo stylecodes, and linear and/or non linear block codes.

In one embodiment of the present invention, the method of FIG. 2 isimplemented on those messages for which there is an advantage inreducing the reception time of the message. In one embodiment of thepresent invention, the method of FIG. 2 is performed when the receiveris in idle mode. Alternatively or additionally, the method of FIG. 2 isimplemented on messages where there is a substantial probability thatthe receiver 20 should go to sleep. Additionally, the method of FIG. 2may be implemented on messages in which there is a substantialprobability that the receiver will go to sleep after receiving themessage even if the message does not relate to the operation of thereceiver 20.

In one embodiment of the present invention, the receiver 20 is a part ofa cellular phone. In this embodiment, the method of FIG. 2 may beimplemented on the messages received on a paging channel. Alternatively,the method of FIG. 2 may be implemented on all the non-audio messages.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method for controlling a receiver, comprising: awakening thereceiver to receive a paging message that is defined by transmittedsymbols that are transmitted in a plurality of separate time slots;demodulating at least a portion of the received symbols; estimating thepaging message using a part of the demodulated symbols that is less thanall of the symbols defining the paging message; assessing a correctnessof the estimated paging message; and if the paging message meets orexceeds a correctness requirement, returning the receiver to a sleepmode based on the paging message having been estimated using fewerdemodulated-symbols than all of the symbols defining the paging message.2. The method of claim 1, wherein estimating the paging messagecomprises decoding the paging message.
 3. The method of claim 1, furthercomprising: obtaining noise data after demodulating the portion of thesymbols and assessing a signal quality of the paging message.
 4. Themethod of claim 3, the step of demodulating further comprising:determining whether to decode an initial portion of the paging messagebased on the signal quality of the paging message.
 5. The method ofclaim 3, the step of demodulating further comprising: demodulatingsymbols in a first pair of time slots; obtaining noise data; anddetermining, based on the noise data, whether to demodulate symbols inan additional time slot.
 6. The method of claim 1, further comprising:demodulating symbols in a first pair of time slots; determining whetheradditional information is needed; and if additional information isneeded, demodulating symbols in a third time slot.
 7. An articlecomprising a computer-readable medium storing instructions that enable aprocessor-based system to: awaken a receiver to receive a paging messagethat is defined by transmitted symbols that are transmitted in aplurality of separate time slots; demodulate at least a portion of thereceived symbols; estimate the paging message using a part of thedemodulated symbols that is less than all of the symbols defining thepaging message; assess a correctness of the estimated paging message;and if the paging message meets or exceeds a correctness requirement,return the receiver to a sleep mode based on the paging message havingbeen estimated using fewer demodulated-symbols than all of the symbolsdefining the paging message.
 8. The article of claim 7, further storinginstructions that enable a processor-based system to: estimate thepaging message by decoding the paging message.
 9. The article of claim7, further storing instructions that enable a processor-based system to:obtain noise data after demodulating the portion of the symbols andassessing a signal quality of the paging message.
 10. The article ofclaim 9, further storing instructions that enable a processor-basedsystem to: determine whether to decode an initial portion of the pagingmessage based on the signal quality of the paging message.
 11. Thearticle of claim 9, further storing instructions that enable aprocessor-based system to: demodulate symbols in a first pair of timeslots; obtain noise data; and determine, based on the noise data,whether to demodulate symbols in an additional time slot.
 12. The methodof claim 7, further storing instructions that enable a processor-basedsystem to: demodulate symbols in a first pair of time slots; determinewhether additional information is needed; and if additional informationis needed, demodulate symbols in a third time slot.
 13. A communicationterminal, comprising: a demodulator configured to receive a pagingmessage that is defined by transmitted symbols that are transmitted in aplurality of separate time slots, and demodulate at least a portion ofthe received symbols; an estimator configured to estimate the pagingmessage using a part of the demodulated symbols that is less than all ofthe symbols defining the paging message; and control circuitryconfigured to determine if the paging message meets or exceeds acorrectness requirement, and if so, return the receiver to a sleep modebased on the paging message having been estimated using fewerdemodulated-symbols than all of the symbols defining the paging message.14. The communication terminal of claim 13, wherein the estimator is adecoder that is configured to decode the paging message using a part ofthe demodulated symbols that is less than all of the symbols definingthe paging message.
 15. The communication terminal of claim 13, whereinthe communication terminal is a cell phone.
 16. The communicationterminal of claim 15, wherein the cell phone is capable of using GSM.17. The communication terminal of claim 13, wherein the controlcircuitry is further configured to: obtain noise data from thedemodulator after the demodulator demodulates the portion of thesymbols; and assess a signal quality of the paging message based on thenoise data.
 18. The communication terminal of claim 17, wherein thecontrol circuitry is further configured to: determine whether to decodean initial portion of the paging message based on the signal quality ofthe paging message.
 19. The communication terminal of claim 17, whereinthe demodulator is configured to demodulate symbols in a first pair oftime slots; and the control circuitry is configured to; obtain noisedata from the demodulator; and determine, based on the noise data,whether to instruct the demodulator to demodulate symbols in anadditional time slot.
 20. The communication terminal of claim 13,wherein the demodulator is configured to demodulate symbols in a firstpair of time slots; and the control circuitry is configured to:determine whether additional information is needed; and if additionalinformation is needed, instruct the demodulator to demodulate symbols ina third time slot.